tox: Drop envdir

tox now always recreates an env although the env is shared using envdir
options.
~~~
$ tox -e genpolicy
genpolicy: recreate env because env type changed from
{'name': 'genconfig', 'type': 'VirtualEnvRunner'} to
{'name': 'genpolicy', 'type': 'VirtualEnvRunner'}
~~~

According to the maintainer of tox, this functionality is not intended
to be supported.
https://github.com/tox-dev/tox/issues/425#issuecomment-1011944293

Change-Id: Ib73364b71234d25d50920495f91e1a3e18c96bcd

tox.ini

@@ -12,7 +12,6 @@ deps =
 commands = stestr run --exclude-regex ".tests.functional" {posargs}
 
 [testenv:functional]
-envdir = {toxworkdir}/shared
 setenv =
   {[testenv]setenv}
 commands =
@@ -25,7 +24,6 @@ deps =
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
-envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 allowlist_externals =
   rm
@@ -36,7 +34,6 @@ commands =
   make -C doc/build/pdf
 
 [testenv:releasenotes]
-envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
@@ -47,7 +44,6 @@ deps =
   -r{toxinidir}/doc/requirements.txt
 
 [testenv:cover]
-envdir = {toxworkdir}/shared
 setenv =
   {[testenv]setenv}
   PYTHON=coverage run --source os_vif,vif_plug_linux_bridge,vif_plug_ovs,vif_plug_noop --parallel-mode
@@ -59,7 +55,6 @@ commands =
   coverage report
 
 [testenv:pep8]
-envdir = {toxworkdir}/shared
 deps =
   hacking>=3.0.1,<3.1.0
 commands = flake8